1. Field of the Invention
The present invention relates to variable ratio drive mechanisms and in particular to such a device suitable for use in bicycles or the like wherein the drive mechanism provides a selectively variable drive ratio.
2. Description of the Prior Art
Multiratio drive mechanisms or transmissions and in particular such devices for use with a bicycle are well known and have been in use for many years. One such device that has gained wide acceptance is a three speed or three ratio device which is mounted within the hub of the rear wheel of a bicycle and which can be manually shifted from one ratio to another by means of a finger operated lever and cable.
In recent years another form of transmission has gained popularity. This transmission is commonly referred to as a "derailler". This transmission comprises broadly a plurality of juxtaposed sprockets mounted to the driven wheel, the pedals, or both, of a bicycle and includes a chain "take-up" device and cable mechanism for causing the bicycle chain to jump from one combination of sprockets to another to thereby provide a plurality of drive ratios. Other forms of multiratio transmissions are also known and typically comprise a multiplicity of sprocket segments mounted to a drive plate. The sprocket segments are arranged in groups to define a plurality of segmented sprockets of different diameters with different groups of the sprocket segments being operated by means of a lever and cable mechanism to cause individual groups of the sprockets to engage the bicycle chain at any one time. The latter two types of bicycle transmissions have the advantage of providing a greater number of drive ratios for a bicycle. However, these latter types of transmissions, when adapted to provide a maximum number of ratios such as, for example, ten ratios, require the use of a substantial number of sprockets or sprocket segments, require the use of two separate shift levers, and have a tendency to allow the bicycle chain to become totally disengaged therefrom if not shifted properly. Further, in the "ten speed" or ten ratio models, a plurality of sprockets must be provided both at the driven wheel and at the pedal portions of the bicycle whereby the transmission is relatively complex.